Amorphous perfluorinated copolymers

ABSTRACT

Amorphous perfluorinated copolymers, comprising cyclic perfluorinated units deriving from at least two different perfluorinated comonomers, optionally with units deriving from a perfluorinated monomer containing at least one olefinic unsaturation (perfluoroolefin), or comprising cyclic perfluorinated units and units deriving from a perfluorinated monomer containing at least one olefinic unsaturation, said perfluorinated copolymers having the following combination of properties: 
     substantial absence of unstable polar end groups, 
     polymer Tg higher than 120° C., 
     narrow monomeric composition distribution.

The present invention relates to amorphous perfluorinated polymers foroptical applications, in particular optical fibers, the preformsobtained therefrom, and a process for preparing said polymers.

With the term “preform” it is generallty meant a compact and transparentsolid formed by polymers having optical properties.

The polymer optical fibers (POF) have a core-sheath structure. Comparedwith the optical fibers wherein the core is based on quartz ormulticomponent glass, POFs generally show the following advantages:

higher diameter,

improved flexibility (according to “bending test”)

improved number opening (difference of the refractive index between coreand sheath),

improved connection easiness to the light source.

Polymer optical fibers having a core of polymethylmethacrylate,polystyrene or polycarbonate and a sheath of aperfluoroalkylmethacrylate polymer are at present on the market. At wavelengths of 400-600 nm polymethylmethacrylate acrylate POFs show a goodattenuation, of the order of 150-400 dB/Km. At wave lengths higher than650 nm the attenuation of said POFs becomes very high. This represents adrawback since on industrial scale sources of light at wave lengthshigher than 700 nm, up to the near infrared, are used.

U.S. Pat. No. 4,966,435 describes POFs having a good transmission to thelight both in the visible spectrum and in the near infrared and anexcellent thermal resistance. In said fibers the core is a copolymer ofa PD or PDD perfluorodioxole. Said copolymers are substantiallyamorphous, have a Tg higher than 100° C., a refractive index which canbe controlled between 1.29 and 1.40 in function of the comonomer, andcan easily be transformed into optical fibers having an uniformdiameter. The thermal resistance of said substantially amorphouscopolymers is high, the number of the unstable end groups being lowerthan 5 meq/Kg, preferably lower than 2 meq/Kg. This reduced number ofend groups is obtained by treating the polymers with amines or alcoholsand subsequently by reacting with fluorine at a temperature lower thanthe polymer Tg and comprised between 20° C. and 200° C. The sheath isformed by a PD/TFE copolymer. The core-sheath fibers obtained accordingto said patent have an attenuation of 270 dB/km at 650 nm, 330 dB/Km at780 nm and 285 dB/km at 1,550 nm. Said values are too high for “lastmile” or LAN (Local Area Network) applications, wherein an attenuationlower than 100 dB/km in the range 700-1,550 nm is required.

EP 710,855 describes graded refractive index POFs having an improvedband amplitude, formed by an amorphous polymer and by at least anotherpolymer (doping agent) having refractive index different of at least0.001 units from that of the amorphous polymer. The doping agent isdistributed in the fiber so as to have a concentration gradient from thecentre to the cable periphery. The fluoropolymer contains cyclicstructures; in particular when alicyclic polymers are used theattenuation is higher than 100 dB/km at a wave length between 700 and1,550 nm. Said attenuation results too high for LAN applications.

EP 752,598 describes graded refractive index POFs similar to those ofprevious patent, wherein a third component, having a specularconcentration gradient with respect to that of the doping agent, isused, to maintain contact the Tg along the fiber diameter. As thirdcomponent a perfluoropolyether having the structure

—CF₂—CF(CF₃)O— or —(CF₂)_(n1)O—

is used, wherein n1 is an integer from 1 to 3. Preferably the numberaverage molecular weight of the perfluoropolyether is from 300 to10,000. In the Examples of said patent it is described that fibershaving a Tg of 82° C. can have an attenuation of 100 dB/Km at 780 nm and65 dB/Km at 1,550 nm. In the comparative Examples it is shown that inabsence of PFPE the attenuation increases to 400 dB/Km at 780 nm, and to280 dB/Km at 1,550 nm, while the Tg ranges from 82° C. in the core to102° C. in the sheath respectively. In absence of PFPE microvacua arepresent in the fibers. The optical fibers obtainable according to saidpatent have the drawback to have a very low Tg for LAN applications,which require a fiber Tg higher than 100° C. (S. Ando et al., ChemTech.Dec. 1994 20-27).

The need was felt to have available amorphous perfluorinated polymersfor optical applications in particular for optical fibers, having thefollowing combination of properties:

attenuation values lower than 100 dB/km at wave lengths from 500 to1,550 nm;

Tg higher than 120° C.;

high thermal resistance;

narrow distribution of the monomeric composition of the amorphousperfluorinated copolymer according to the following test: the polymer isdissolved at a concentration of 5% w/w in perfluoroheptane solvent,after filtration on PTFE filter having a maximum pore diameter of 0.2micron, no polymer residue remains on the filter; the filter weightbefore and after filtration remains unchanged.

The Applicant has unexpectedly and surprisingly found perfluorinatedamorphous polymers which solve this technical problem.

An object of the present invention are amorphous perfluorinatedcopolymers, comprising cyclic perfluorinated units deriving from atleast two different perfluorinated comonomers, optionally with unitsderiving from a non cyclic perfluorinated monomer, or which does notcyclize during the polymerization, containing at least one olefinicunsaturation (perfluoroolefin), or comprising cyclic perfluorinatedunits and units deriving from a non cyclic perfluorinated monomer, orwhich does not cyclize during the polymerization, containing at leastone olefinic unsaturation, said perfluorinated copolymers having thefollowing combination of properties:

substantial absence of unstable polar end groups, in particular endgroups as COF, COOH or their amidic derivatives, esters or salts; saidend groups being not detectable in the copolymer, i.e. present in anamount lower than 0.05 mmoles/kg polymer when they are determined byFourier transform IR spectroscopy by Nicolet® Nexus FT-IR equipment (256scannings, resolution 2 cm⁻¹), wherein:

on a sintered polymer powder pellet having 5 mm diameter and thicknessfrom 50 to 300 micron (corresponding to a weight of 1.75 and 10.5 mg ofpolymer, respectively) a scanning between 4,000 cm⁻¹ and 400 cm⁻¹ isinitially carried out, the pellet being then kept for 12 hours in anenvironment saturated with ammonia vapours;

the IR spectrum is finally recorded under the same conditions of theinitial IR spectrum;

the two spectra are elaborated by subtracting from the signals of thespectrum relating to the untreated sample (initial spectrum) thosecorresponding to the specimen spectrum after exposure to ammoniavapours;

the “difference” spectrum is obtained, which is normalized by thefollowing equation:$\frac{``{{Difference}\quad {spectrum}}"}{\left\lbrack {{pellet}\quad {weight}\quad {(g)/{pellet}}\quad {area}\quad \left( {cm}^{2} \right)} \right\rbrack}$

 the optical densities related to the end groups which have reacted withthe ammonia vapours are measured, which give rise to detectable peaks inthe IR spectrum, said end groups being the COOH and COF groups;

the optical densities are converted in mmoles/kg polymer using theextinction coefficients reported in Table 1, page 73 of the paper by M.Pianca et Al. “End groups in fluoropolymers”, J. Fluorine Chem. 95(1999), 71-84 (herein incorporated by reference); the so found valuesgive the concentrations of the residual polar end groups as mmoles ofpolar end groups/kg polymer: in the spectrum of the amorphous(per)fluorinated polymers after fluorination bands related to COOHgroups (3,600-3,500, 1,820-1,770 cm⁻¹) and/or COF groups (1,900-1,830cm⁻¹) are not detectable, the method detectability limit being 0.05mmoles/Kg polymer;

polymer Tg higher than 120° C., preferably 125° C.; narrow distributionof the monomeric composition of the amorphous perfluorinated copolymermeasured according to the following test: the polymer is dissolved at aconcentration of 5% w/w in perfluoroheptane solvent, after filtration onPTFE filter having a maximum pore diameter of 0.2 micron, no polymerresidue remains on the filter whereby the filter weight before and afterfiltration remains unchanged.

The amorphous perfluorinated polymers according to the present inventiondo not substantially contain C—H bonds.

The perfluorinated monomers which give polymers containing cyclicstructures, are selected from the following:

perfluorodioxoles, wherein optionally one or two fluorine atoms aresubstituted with chlorine atoms;

non conjugated dienes having the following formulas:

CF₂═CFOCF₂CF₂CF═CF₂,

CFX¹═CX²OCX³X⁴OCX²═CX¹F

wherein X¹ and X², equal to or different from each other, are F, Cl; X³and X⁴, equal to or different from each other, are F or CF₃, whichduring the polymerization cyclopolymerize.

The preferred perfluorodioxoles are those having the following formula:

wherein R′_(F) is equal to F, R_(F) or OR_(F) wherein R_(F) is a linearor branched when possible perfluoroalkyl radical having 1-5 carbonatoms; X₁ and X₂ equal to or different from each other being F or CF₃.

Preferably in formula (IA) R′_(F)=OR_(F), R_(F) is preferably CF₃;X₁=X₂=F, and the compound is known as TTD. Another preferred cyclicmonomer is that having formula (IA) wherein R′_(F)=F; X₁=X₂=F, and thecompound is known as PD.

The non cyclic perfluorinated comonomers, or which do not cyclize duringthe polymerization, containing at least one ethylene unsaturation, areselected from the following:

C₂-C₈ perfluoroolefins, such as TFE, hexafluoropropene (HFP);

(per)fluoroalkylvinylethers (PAVE) CF₂═CFOR_(f), wherein R_(f) is aC₁-C₆ (per)fluoroalkyl, for example CF₃, C₂F₅, C₃F₇;

perfluoro-oxyalkylvinylethers CF₂═CFOX, wherein X is: a C₁-C₁₂ alkyl, ora C₁-C₁₂ oxyalkyl, or a C₁-C₁₂ perfluoro-oxyalkyl having one or moreether groups, for example perfluoro-2-propoxy-propyl.

The perfluorinated polymers of the invention can contain also as noncyclic comonomer or which does not cyclize during the polymerization,containing one ethylene unsaturation, in substitution or in addition tothe above comonomers, also non cyclic C₂-C₈, fluoroolefins or which donot cyclize during the polymerization, containing one or two chlorineatoms, such as chlorotrifluoroethylene (CTFE).

When the amorphous polymer optionally contains units deriving from a noncyclic perfluorinated monomer, or which does not cyclize during thepolymerization, containing at least one olefinic unsaturation, itcontains cyclic units deriving from either different cyclic monomers ora cyclic monomer and a monomer which cyclizes during the polymerization.

The amounts expressed in per cent by moles of cyclic units deriving fromeither different cyclic monomers or a cyclic monomer and a monomer whichcyclizes during the polymerization range from 60% to 100% by moles,preferably from 80% to 100%.

When the amorphous polymer contains units deriving from a non cyclicperfluorinated monomer or which does not cyclize during thepolymerization and containing at least one olefinic unsaturation, itcontains cyclic units deriving from a cyclic monomer, or from twodifferent cyclic monomers, or from a cyclic monomer and a monomer whichcyclizes during the polymerization.

The amounts expressed in per cent by moles of cyclic units deriving froma cyclic monomer, or from two different cyclic monomers or from a cyclicmonomer and a monomer which cyclizes during the polymerization rangefrom 60% to 99.5%, preferably from 80% to 95%.

When the copolymer contains units deriving from a non cyclicperfluorinated monomer or which does not cyclize during thepolymerization and containing at least one olefinic unsaturation, it canoptionally be present a second non cyclic perfluoroolefin or which doesnot cyclize during the polymerization, different from that alreadypresent, selected from the above non cyclic perfluoroolefins or which donot cyclize in polymerization, in amounts in per cent by moles from 0 to20%, preferably lower than 10%.

The preferred copolymers according to the present invention are thecopolymers obtained by copolymerizing:

the perfluorodioxoles of formula (IA) having TTD structure, with TFE;

the perfluorodioxoles of formula (IA) having TTD structure, withperfluorodioxoles of formula (IA) having PD structure.

The dioxoles and their copolymers, before being subjected to the processaccording to the present invention, as described hereafter, are preparedby polymerization for example according to U.S. Pat. Nos. 5,498,682 and5,883,177. The amorphous polymers can be prepared according topolymerization methods in emulsion, suspension and bulk according toknown methods of the prior art. In particular amorphous polymers can beprepared by monomer copolymerization in aqueous emulsion, according toknown methods of the prior art, in the presence of radical initiatorsfor example alkaline or ammonium persulphates, perphosphates, perboratesor percarbonates, optionally in combination with ferrous, cupreous orsilver salts, or other easily oxidizable metals. In the reaction mediumalso anionic, cationic and non ionic, hydrogenated and fluorinatedsurfactants are usually present, among which fluorinated surfactant areparticularly preferred. The polymerization reaction is generally carriedout at temperatures in the range 25°-150° C., using a pressure up to 10MPa. The preparation of amorphous perfluorinated copolymers ispreferably carried out in microemulsion of (per)fluoropolyoxyalkylenes,according to U.S. Pat. Nos. 4,789,717 and 4,864,006. Optionally inpolymerization well known chain transfer agents of the prior art canalso be used.

The determination of the narrow distribution of he monomeric compositionof the amorphous perfluorinated polymer of the invention, as said, iscarried out according to the following wing test: the polymer isdissolved at a 5% concentration w/w in perfluoroheptane, after havingfiltered the solution on PTFE filter having a maximum pore diameter of0.2 micron, no polymer residue remains on the filter; in other terms thefilter weight before and after filtration remains unchanged.

A further object of the present invention is a process for preparing theabove perfluorinated polymers, including also the homopolymers formed bycyclic units deriving even from only one cyclic monomer or from amonomer which cyclizes during the polymerization, comprising thefoollowing steps:

I) dissolution of the amorphous perfluorinated polymer in a solventinert to fluorination, having solubility parameter defined as in PolymerHandbook 3th ed. page VII 519, (Ed. Brandrup and Immergut), from 5.5 to8.2 (cal/cm³)^(1/2), the polymer being dissolved at a concentration,expressed in per cent by weight, from 0.5% to 15%, preferably from 3% to8%;

II) polymer fluorination in solution by treatment with elementaryfluorine, optionally in admixture with inert gases, in the presence ofultraviolet radiations having a wave length from 200 to 500 nm, attemperatures lower than 100° C.;

III) solution filtration on PTFE filter having a maximum pore diameterof 0.2 μm, recovery of the filtrate; the filtrate contains the amorphousperfluorinated polymers of the present invention usable for opticalapplications;

IV) optionally, solvent removal and recovery of the amorphousperfluorinated polymers usable for optical applications. In step I) onemust use, as said, at least a solvent inert to fluorination, havingsolubility parameter defined as in Polymer Handbook 3th ed. page VII519, (Ed. Brandrup and Immergut), from 5.5 to 8.2 (cal/cm³)^(1/2).Solvents which satisfy the solubility parameter are preferably selectedfrom perfluoroheptane, known as Galden®D80,perfluorobutyl-tetrahydrofuran, perfluoropropyl-tetrahydropyran,perfluoropolyether having b.p. 165° C., known as Galden®LS 165, tertiaryperfluoroamines, etc.

In step I) to the organic solution also a doping compound can optionallybe added, in an amount expressed in per cent by weight on theperfluoropolymer, from 1% to 30%, preferably from 5% to 20%. With dopingcompound (see hereunder) a substance is meant whose refractive indexdiffers from that of the perfluoropolymer of at least 0.001 units,preferably of 0.005 units.

In step II) the used radiation has a wave length ranging from 200 to 500nm; a mercury lamp Hanau TQ 150 can for example be used. By fluorinationthe amount of polar end groups in the polymer is reduced and resultsundetectable with the above described infrared spectroscopy method.Therefore their residual amount is lower than the method detectabilitylimit which, as said, is of 0.05 mmoles/kg polymer. The reactiontemperature in step II) ranges from 0° C. to <+100° C., preferably from+20° C. to +50° C.

As said, the determination of the polar end groups before and afterfluorination is carried out by IR spectroscopy, performing a scanningbetween 4,000 cm⁻¹ and 400 cm⁻¹, on a sintered polymer powder pelletwhich can have a thickness from 50 to 300 micron. The fluorinationprocess ends when by IR spectroscopy bands related to COOH groups(3,600-3,500, 1,820-1,770 cm⁻¹) and/or COF groups (1,900-1,830 cm⁻¹) areno longer detectable. The method detectability limit is 0.05 mmoles/Kgpolymer.

In step III) the temperature at which the filtration is carried out isin the range 0°-100° C., preferably it is the room temperature (20°-25°C.).

In step IV) the solvent is removed by means of known methods, forexample by stripping under vacuum.

The amorphous perfluorinated polymer obtained at the end of the processaccording to the present invention, shows the combination of the aboveproperties:

attenuation values lower than 100 dB/km at wave lengths from 500 to1,550 nm;

Tg higher than 120° C.;

high thermal resistance;

narrow distribution of the monomeric composition as above defined.

The amorphous perfluorinated polymer obtained at the end of the processaccording to the present invention when step IV) is carried out, can beprocessed to obtain the preform. In this case the polymer is introducedin a vessel, for example of glass or porcelain, or metal coated bysemi-crystalline fluoropolymers, such for example PFA, FEP, PTFE, thenthe temperature is increased over the fluoropolymer Tg, preferably tovalues from 50° C. to 100° C. higher than the fluoropolymer Tg. Thetemperature is kept for a time sufficient to allow the polymercompacting by gravity, or by applying a pressure. Optionally it ispossible to use vacuum and pressure cycles. At the end of the treatmentthe temperature is gradually lowered to room temperature. In said waythe preform of the amorphous perfluorinated polymer usable for opticalapplications is obtained.

The preform can optionally contain, in a total amount expressed in percent by weight on the perfluoropolymer, from 1% to 30%, preferably from5% to 20% of additives, preferably doping agents and/orperfluoropolyethers.

The used doping agents are fluorinated compounds substantially C—H bondsfree and can be selected from the group comprising compounds having alow molecular weight, oligomers and polymers containing an aromaticring, for example benzenic or naphthalenic, and/or one or more halogenatoms, selected from Cl, Br and Iodine, or a functional group such forexample a ketonic group or an ether function. Examples of dopants are:1,3-dibromo-tetrafluorobenzene, 1,4-dibromo-tetrafluorobenzene,2-bromotetrafluorobenzotrifluoride, halogenpentafluorobenzene whereinthe halogen is as above; decafluorobenzophenone, perfluoroacetophenone,perfluorodiphenyl, chloroheptafluoronaphthalene,bromoheptafluoronaphthalene; chlorotrifluoroethylene oligomers withoptionally a second perfluoroolefinic monomer selected from the abovefluoropolymer non cyclic monomers.

Other optional components that the preform can contain areperfluoropolyethers containing in the chain one or more of the followingunits: —CF₂—CF(CF₃)O—, —CF(CF₃)—CF₂O—, —(CF₂)_(n1)O— wherein n1 is aninteger from 2 to 3, —CFX₁O— wherein X₁=F, CF₃ having number averagemolecular weight preferably from 300 to 10,000, still more preferablyfrom 500 to 2,500, which can be used as doping agents and as Tgregulators. The perfluoropolyether compounds are well known in the priorart and on the market.

The homopolymers formed by cyclic units deriving even from only onecyclic monomer or from a momomer which cyclizes during thepolymerization, obtainable with the invention process, result novel withrespect to the homopolymers available on the market but which have notbeen subjected to fluorination and filtration steps according to thepresent invention.

The polymers obtainable with the process of the present invention andthe corresponding preforms optionally comprising also the other abovecompounds (doping compounds and perfluoropolyethers) are characterizedby a high transparency at wave lengths from 500 to 1,550 nm, and areusable for preparing manufactured articles for optical applications,such for example optical wave guides, optical components, optical lensand windows, optical fibers, graded index and step index.

Tests carried out by the Applicant have shown that by increasing thewave length from 400 to 1,550 nm the attenuation tends to decrease.Therefore on the basis of the attenuation value measured at 514 nm, asindicated in the Examples of the present invention, the attenuation athigher wave lengths is equal to or lower than the values measured at 514nm.

The following Examples illustrate the invention without limiting thescope thereof.

EXAMPLES

Attenuation Measurement Method

The attenuation calculation has been effected according to the methodreported in Koike et Al., Macromolecules, vol. 25 page 4807, 1992.

The polymer under the form of compact transparent solid cylinder, issubjected to scattering measurements by the Laser Light scattering (LLS)technique. The LLS measurement is carried out on the polymer cylinderimmersed in distilled water in an optical glass test tube immersed intoluene. The wave length is 514.5 nm, the laser power has been fixed at90 mW with a detectability pinhole of 400 μm. The specimen is kept underrotation at 25 rpm to allow a space average during the measurement. Thescattering has been measured at 90°, with acquisition times of 0.5 s.The attenuation is calculated considering the scattering intensityindependent of the angle. In the calculation only the vertical componentof the scattered light has been considered.

Polar (Ionic) End Group Determination by IR Spectroscopy

A small amount of the specimen (3.5 mg) is put under a press (3.5ton/cm²) and a pellet having a 5 mm diameter and 100 micron thickness isprepared.

The spectrum from 4,000 to 400 cm⁻¹ is recorded by Nicolet® Nexus FT-IRequipment (250 scannings, resolution 2 cm⁻¹).

The pellet is transferred in an environment saturated with ammoniavapours. After 12 hours the specimen is taken from the ammonia saturatedenvironment and the IR spectrum is recorded again under the sameconditions.

By subtracting from the signals of the spectrum relating to the specimenas such those corresponding of the specimen spectrum after exposure toammonia vapours, the “difference” spectrum is obtained, which isnormalized by the following equation:$\frac{``{{Difference}\quad {spectrum}}"}{\left\lbrack {{pellet}\quad {weight}\quad {(g)/{pellet}}\quad {area}\quad \left( {cm}^{2} \right)} \right\rbrack}$

The optical densities related to the end groups which have reacted withthe ammonia vapours are measured. They are generally COOH or COF groups.The optical densities are transformed into mmoles/kg polymer using theextinction coefficients shown in Table 1, page 73 of the paper by M.Pianca et Al. “End groups in fluoropolymers”, J. Fluorine Chem. 95(1999), 71-84. The found values express the concentrations of theresidual polar end groups as mmoles of polar end groups/kg polymer. Thedetectability limit is 0.05 mmoles/Kg polymer.

Polymer Tg Determination

The Tg is measured by DSC.

Test of Narrow Distribution of the Monomeric Composition

The test is carried out with the following procedure: the polymer isdissolved at 5% concentration w/w in perfluoroheptane having asolubility parameter 5.8, and the obtained solution is filtered on PTFEmembrane having a maximum pore diameter of 0.2 micron. After filtration,no polymer residue has to remain on the filter whereby the filter weightbefore and after filtration must remain unchanged.

Example 1

Preparation of Copolymer TFE/TTD 20/80

In a 5 l AISI 316 autoclave, equipped with stirrer working at 650 rpm,after vacuum has been made by oil pump, are introduced in sequence 2,790ml of demineralized water, 6.67 g/liter H₂O of microemulsion obtained bymixing:

7.5 ml of a perfluoropolyoxyalkylene having acid end group of formula:

CF₂ClO(CF₂—CF(CF₃)O)_(n)(CF₂O)_(m)CF₂COOH

 wherein n/m=10, having average molecular weight 600;

7.5 ml of an aqueous NH₃ solution at 30% by volume;

15 ml of demineralized water;

4.5 ml of Galdeno® D02 of formula:

CF₃O(CF₂—CF(CF₃)O)_(n)(CF₂O)_(m)CF₃

wherein n/m=20, having average molecular weight 450.

The autoclave is heated up to 75° C. and at this temperature 33.3g/liter H₂O of TTD are added. The pressure inside the autoclave isbrought to 1.4 MPa by TFE gas. Subsequently 210 ml of a solution 0.0925M of potassium persulphate are introduced. After about 10 minutes thereaction starts. The reaction pressure is kept constant by feeding in asemicontinuous way, at each decrease of the inner pressure of 0.05 MPa,liquid TTD and gaseous TFE in the ratio by weight TTD/TFE=1.4. Thereaction is stopped after having fed in the whole about 320 g of TTD.The latex is degassed and it has a 15% by weight concentration of solid.It is coagulated by adding HNO₃ at 65% w/w, the polymer is separatedfrom the aqueous phase, it is washed twice with demineralized water anddried in a stove at 85° C. for 100 h.

The polymer Tg is about 134.7° C. corresponding to a composition bymoles 20/80 TFE/TTD. The intrinsic viscosity measured at 30° C. inperfluoroheptane (Galdeno®D80) is equal to 36.6 cc/g. By IR analysis thepolymer results to contain a residual amount of the polar COOH groups of6 mmoles/Kg polymer.

Example 2

Preparation of a copolymer TFE/TTD 20/80 for optical applicationsaccording to the process of the present invention and preformachievement

255 grams of the copolymer of Example 1, in the form of white powder aredissolved in 3,000 ml of Galden® D80 maintaining the stirring for 2hours. The solution is degassed by feeding nitrogen for two hours at 22°C. in a 5 l photochemical glass reactor equipped with mechanical stirrerand a mercury vapour immersion UV lamp (Hanau TQ 150) and then subjectedto fluorination for 31 h at 35° C. by feeding a mixture 1:1 by volume ofnitrogen/fluorine in the presence of the UV radiation. The solutionafter fluorination is filtered on PTFE membranes having a maximum porediameter of 0.2 μm by a filter under pressure at 0.15 MPa.

The compound remained on the filter has been characterized by IRspectrum and DSC. The IR spectrum shows that it is a fluorinated polymercontaining TTD. The DSC measurement shows for the fluorinated polymerremained on the filter a Tg of 100° C., corresponding to a compositionon a molar basis 50/50 TFE/TTD.

The polymer IR analysis has shown that the concentration of the polargroups is lower than 0.05 mmoles/Kg polymer (method detectabilitylimit).

The solvent is removed from the filtered solution by evaporation undervacuum with a temperature gradient starting from 50° C. up to 280° C.,until obtaining the dry polymer.

The polymer is introduced in a test tube having a lenght of 20 cm and adiameter of 2 cm, and kept liquid at the temperature of 280° C. for onehour, then it is slowly cooled, with a decreasing temperature profile,to room temperature in a time of 12 h. A cylinder having sizes of 2cm×5.2 cm (preform) is obtained.

The polymer cylinder has been subjected to scattering measurements bythe Laser Light scattering (LLS) technique as above described. Theattenuation is equal to 17×10⁻⁵ dB/cm (17 dB/km) at 514 nm.

The polymer IR analysis has shown that the concentration of the polargroups is lower than 0.05 mmoles/Kg polymer (method detectabilitylimit).

The polymer dissolved at a concentration of 5% by weight in Galden® D80and filtered on a PTFE membrane having a maximum pore diameter of 0.2 μmdoes not leave residues on the filter. The filter, dried at 150° C. for1 h and weighed by a precision balance has the same weight as theinitial weight.

Example 3 (Comparative)

Fluorination of a Copolymer TFE/TTD 20/80

250 g of the polymer prepared according to Example 1 have been treatedas in Example 2, omitting the filtration on PTFE membrane having poresof 0.2 μm.

The polymer IR analysis has shown that the concentration of polar groupsis lower than 0.05 mmoles/Kg polymer (method detectability limit).

A cylinder having 2 cm×5.2 cm sizes is obtained.

The polymer cylinder has been subjected to scattering measurements bythe Laser Light scattering (LLS) technique as above described. Theattenuation is 0.00927 dB/cm (927 dB/km) at 514 nm.

The polymer dissolved at a concentration of 5% by weight in Galden® D80and filtered on a PTFE membrane having a maximum pore diameter of 0.2 μmleaves an evident polymer residue on the filter equal to 1.69 g.

The compound remained on the filter has been characterized by IRspectrum and DSC. The IR spectrum shows that it is a fluorinated polymercontaining TTD. The DSC measurement shows for the fluorinated polymerremained on the filter a Tg of 100° C., corresponding to a compositionon a molar basis 50/50 TFE/TTD.

Example 4 (Comparative)

Filtration of a Copolymer TFE/TTD 20/80

Example 2 is repeated omitting only the fluorination step.

The polymer IR analysis has shown that the concentration of polar groupsis of 6 mmoles/Kg polymer.

A cylinder having 2 cm×5.2 cm sizes is obtained.

The polymer cylinder has been subjected to scattering measurements bythe Laser Light scattering (LLS) technique as above. The attenuation isequal to 0.06 dB/cm (6,000 dB/km) at 514 nm.

The polymer dissolved at a concentration of 5% by weight in Galden®D80and filtered on a PTFE membrane having a maximum pore diameter of 0.2 μmdoes not leave residues on the filter. The filter, dried at 150° C. for1 h and weighed by a precision balance has the same weight as theinitial weigh.

Example 5

Preparation of Copolymer TFE/TTD 20/80

The preparation of the copolymer of Example 1 is repeated by obtaining apolymer having a Tg of about 133.1° C. The intrinsic viscosity, measuredat the temperature of 30° C. in perfluoroheptane is 35.2 ml/g. By IRanalysis the polymer results to contain a residual amount of the polarCOOH groups of 6 mmoles/Kg polymer.

Example 6

Preparation of a copolymer for optical applications according to theprocess of the present invention, and achievement of the preform, byusing the amorphous perfluorinated polymer prepared in Example 5.

Example 2 is repeated, by using the polymer of Example 5, except thefluorination time, which is 42 h instead of 31 h.

The compound remained on the filter has been characterized by IRspectrum and DSC. The IR spectrum shows that it is a fluorinated polymercontaining TTD. The DSC measurement shows for the fluorinated polymerremained on the filter a Tg of 100° C., corresponding to a compositionon a molar basis 50/50 TFE/TTD.

The polymer IR analysis has shown that the concentration of the polargroups is lower than 0.05 mmoles/Kg polymer (method detectabilitylimit).

The solvent is removed as described in Example 2 and the polymer treatedas described until obtaining a cylinder having 2 cm×4.3 cm sizes(preform).

The polymer cylinder has been subjected to scattering measurements bythe Laser Light scattering (LLS) technique as above described. Theattenuation is equal to 26×10⁻⁵ dB/cm (26 dB/krm) at 514 nm.

The polymer dissolved at a concentration of 5% by weight in Galden®D80and filtered on a PTFE membrane having a maximum pore diameter of 0.2 μmdoes not leave residues on the filter. The filter, dried at 150° C. for1 h and weighed by a precision balance has the same weight as theinitial weight.

Example 7

Preparation of Copolymer TFE/TTD 20/80

The preparation of the copolymer of Example 1 is repeated obtaining apolymer having a Tg of about 137.4° C. The intrinsic viscosity, measuredat the temperature of 30° C. in perfluoroheptane is 40.0 ml/g. By IRanalysis the polymer results to contain a residual amount of the polarCOOH groups of 6 mmoles/kg polymer.

Example 8

Preparation of a copolymer for optical applications according to theprocess of the present invention, and achievement of the preform, usingthe amorphous perfluorinated polymer prepared in Example 7.

Example 2 is repeated, by using the polymer of Example 7, with afluorination time of 30 h.

The polymer IR analysis has shown that the concentration of the polargroups is lower than 0.05 mmoles/Kg polymer (method detectabilitylimit).

In the preform preparation step the polymer is introduced in a test tubehaving a length of 20 cm and a diameter of 2 cm and kept liquid at thetemperature of 280° C. for one hour, then slowly cooled, with adecreasing temperature profile, to room temperature in a time of 24 h. Acylinder having 2 cm×6.34 cm sizes is obtained.

The polymer cylinder has been subjected to scattering measurements bythe Laser Light scattering (LLS) technique as above. The attenuation isequal to 12×10 cm⁻⁵ dB/cm (12 dB/km) at 514 nm.

The polymer dissolved at a concentration of 5% by weight in Galden®D80and filtered on a PTFE membrane having a maximum diameter of 0.2 μm doesnot leave residues on the filter. The filter, dried at 150° C. for 1 hand weighed by a precision balance has the same weight as the initialweight.

What is claimed is:
 1. Amorphous perfluorinated copolymers, comprising:A) cyclic perfluorinated units deriving from at least two differentperfluorinated comonomers, optionally with units deriving from a noncyclic perfluorinated monomer, or which does not cyclize during thepolymerization, wherein said amorphous perfluorinated copolymer containsat least one olefinic unsaturation, or B) cyclic perfluorinated unitsand units deriving from a non cyclic perfluorinated monomer, or whichdoes not cyclize during the polymerization, wherein said amorphousperfluorinated copolymer contains at least one olefinic unsaturation,and wherein said amorphous perfluorinated copolymer has the followingcombination of properties: substantial absence of unstable polar endgroups; said end groups being not detectable in the copolymer, i.e.present in an amount lower than 0.05 mmoles/kg polymer when they aredetermined by Fourier transform IR spectroscopy by Nicolet® Nexus FT-IRequipment at 256 scannings, resolution 2 cm⁻¹, wherein: on a sinteredpolymer powder pellet having 5 mm diameter and thickness from 50 to 300micron, corresponding to a weight of 1.75 and 10.5 mg of polymer,respectively, a scanning between 4,000 cm⁻¹ and 400 cm⁻¹ is initiallycarried out, the pellet being then kept for 12 hours in an environmentsaturated with ammonia vapours; the IR spectrum is finally recordedunder the same conditions of the initial IR spectrum; the two spectraare elaborated by subtracting from the signals of an initial spectrumrelating to the untreated sample those corresponding to the specimenspectrum after exposure to ammonia vapours; the “difference” spectrum isobtained, which is normalized by the following equation:$\frac{``{{Difference}\quad {spectrum}}"}{\left\lbrack {{pellet}\quad {weight}\quad {(g)/{pellet}}\quad {area}\quad \left( {cm}^{2} \right)} \right\rbrack}$

 the optical densities related to the end groups which have reacted withthe ammonia vapours are measured, which give rise to detectable peaks inthe IR spectrum, said end groups being the COOH and COF groups; theoptical densities are converted in mmoles/kg polymer using theextinction coefficients shown in Table 1, page 73 of the paper by M.Pianca et al., “End groups in fluoropolymers”, J. Fluorine Chem. 95(1999), 71-84 the so found values give the concentrations of theresidual polar end groups as mmoles of polar end groups/kg polymer: inthe spectrum of the amorphous (per)fluorinated polymers afterfluorination bands between 3,600-3,500 or 1,820-1,770 cm⁻¹ related toCOOH groups and/or bands between 1,900-1,830 cm⁻¹ related to COF groupsare not detectable, the method detectability limit being 0.05 mmoles/Kgpolymer; polymer Tg higher than 120° C., narrow distribution of themonomeric composition of the amorphous perfluorinated copolymer measuredaccording to the following test: the polymer is dissolved at aconcentration of 5% w/w in perfluoroheptane solvent, after filtration onPTFE filter having a maximum pore diameter of 0.2 micron, no polymerresidue remains on the filter whereby the filter weight before and afterfiltration remains unchanged.
 2. Amorphous perfluorinated copolymersaccording to claim 1, wherein the perfluorinated monomers which givepolymers containing cyclic structures, are selected from the following:perfluorodioxoles, wherein optionally one or two fluorine atoms aresubstituted with chlorine atoms; non conjugated dienes having thefollowing formulas: CF₂═CFOCF₂CF₂CF═CF₂, CFX¹═CX²OCX³X⁴OCX²═CX¹F whereinX¹ and X², equal to or different from each other, are F, Cl; X³ and X⁴,equal to or different from each other, are F or CF₃, which during thepolymerization cyclopolymerize.
 3. Amorphous perfluorinated copolymersaccording to claim 2, wherein the perfluorodioxoles have the followingformula:

wherein R′_(F) is equal to F, R_(F) or OR_(F) wherein R_(F) is a linearor branched perfluoroalkyl radical having 1-5 carbon atoms; X₁ and X₂equal to or different from each other being F or CF₃.
 4. Amorphousperfluorinated copolymers according to claim 3, wherein in formula (IA)R′_(F)═OR_(F); X₁=X₂=F (TTD).
 5. Amorphous perfluorinated copolymersaccording to claim 4, wherein R_(F) is CF₃.
 6. Amorphous perfluorinatedcopolymers according to claim 3, wherein in formula (IA) R′_(f)═F;X₁=X₂=F (PD).
 7. Amorphous perfluorinated copolymers according to claim1, wherein the non cyclic perfluorinated comonomers, or which do notcyclize during the polymerization, containing at least one ethyleneunsaturation, are selected from the following: C₂-C₈ perfluoroolefins;(per)fluoroalkylvinylethers (PAVE) CF₂═CFOR_(f), wherein R_(f) is aC₁-C₆ (per)fluoroalkyl; perfluoro-oxyalkylvinylethers CF₂═CFOX, whereinX is a C₁-C₁₂ perfluoro-oxyalkyl having one or more ether groups. 8.Amorphous perfluorinated copolymers according to claim 7, wherein theC₂-C₈ perfluoroolefins are TFE or hexafluoropropene (HFP).
 9. Amorphousperfluorinated copolymers according to claim 7, wherein the C₁-C₆(per)fluoroalkyl is CF₃, C₂F₅ or C₃F₇.
 10. Amorphous perfluorinatedcopolymers according to claim 7, X is-perfluoro-2-propoxy-propyl. 11.Amorphous perfluorinated copolymers according to claim 1, comprising noncyclic C₂-C₈ fluoroolefins, or which do not cyclize during thepolymerization, containing one or two chlorine atoms.
 12. Amorphousperfluorinated copolymers according to claim 11, wherein said non cyclicC₂-C₈ fluoroolefins are chlorotrifluoroethylene (CTFE).
 13. Amorphousperfluorinated copolymers according to claim 1, wherein when theamorphous polymer optionally contains units deriving from a non cyclicperfluorinated monomer, or which does not cyclize during thepolymerization, containing at least one olefinic unsaturation, itcontains cyclic units deriving from either different cyclic monomers ora cyclic monomer and a monomer which cyclizes during the polymerization.14. Amorphous perfluorinated copolymers according to claim 13, whereinthe amount expressed in percent by moles of cyclic units deriving eitherfrom different cyclic monomers or from a cyclic monomer and a monomerwhich cyclizes during the polymerization ranges from 60% to 100% bymoles.
 15. Amorphous perfluorinated copolymers according to claim 14,wherein the amount expressed in percent by moles of cyclic unitsderiving either from different cyclic monomers or from a cyclic monomerand a monomer which cyclizes during the polymerization ranges from 80%to 100% by moles.
 16. Amorphous perfluorinated copolymers according toclaim 1, wherein when the amorphous polymer contains units deriving froma non cyclic perfluorinated monomer, or which does not cyclize duringthe polymerization, containing at least one olefinic unsaturation, itcontains cyclic units deriving from a cyclic monomer, or from twodifferent cyclic monomers, or from a cyclic monomer and a monomer whichcyclizes during the polymerization.
 17. Amorphous perfluorinatedcopolymers according to claim 16, wherein the amount expressed inpercent by moles of cyclic units deriving from a cyclic monomer, or fromtwo different cyclic monomers or from a cyclic monomer and a monomerwhich cyclizes during the polymerization ranges from 60% to 99.5%. 18.Amorphous perfluorinated copolymers according to claim 17, wherein theamount expressed in percent by moles of cyclic units deriving from acyclic monomer, or from two different cyclic monomers or from a cyclicmonomer and a momomer which cyclizes during the polymerization rangesfrom 80% to 95% by moles.
 19. Amorphous perfluorinated copolymersaccording to claim 16, optionally comprising a second non cyclicperfluoroolefin or which does not cyclize during the polymerization,different from that already present, selected from the group consistingof C₂-C₈ perfluoroolefins; non-cyclic C₂-C₈ fluoroolefins containing oneor two chlorine atoms in amounts in percent by moles from 0 to 20%. 20.Amorphous perfluorinated copolymers according to claim 19, wherein theC₂-C₈ perfluoroolefins are TFE or hexafluoropropene (HFP).
 21. Amorphousperfluorinated copolymers according to claim 19, the non-cyclic C₂-C₈fluoroolefins containing one or two chlorine atoms arechlorotrifluoroethylene (CTFE).
 22. Amorphous perfluorinated copolymersaccording to claim 19, wherein the second non cyclic perfluoroolefin orwhich does not cyclize during the polymerization is in amounts lowerthan 10% by moles.
 23. Amorphous perfluorinated copolymers according toclaim 1, obtainable by copolymerizing: the perfluorodioxoles of formula(IA) having TTD structure, with TFE; the perfluorodioxoles of formula(IA) having TTD structure, with perfluorodioxoles of formula (IA). 24.Amorphous perfluorinated copolymers according to claim 23, obtainable bycopolymerizing the perfluorodioxoles of formula (IA) having TTDstructure with PD.
 25. Amorphous perfluorinated copolymers according toclaim 1, wherein the unstable polar end groups are COF, COOH or theiramidic derivatives, esters or salts.
 26. Amorphous perfluorinatedcopolymers according to claim 1, wherein the polymer Tg is 125° C.